1. Field of the Invention
The present invention relates to a medical image conversion apparatus and method for converting time series medical images for representing the motion of an organ such as a heart, for example, to display them by volume rendering. Further, the present invention relates to a program for causing a computer to execute the medical image conversion method.
Further, the present invention relates to a medical image conversion apparatus and method for converting a series of plural medical images that have been obtained, for example, by imaging using a contrast agent, and the signal values of which change as time passes, to display the images by volume rendering, or the like. Further, the present invention relates to a program for causing a computer to execute the medical image conversion method.
2. Description of the Related Art
In recent years, high-quality three-dimensional images became used in image-based diagnosis because of an advance of medical equipment (for example, a multi-detector CT, or the like). Here, the three-dimensional image is composed of many two-dimensional tomographic images, and the information amount of the three-dimensional image is large. Therefore, in some cases, doctors need a time to find a desired observation region and to perform diagnosis on the region. Therefore, various techniques have been proposed to improve a characteristic of visually recognizing a whole structure, and also a lesion included in the structure. The visual recognition characteristic is improved by identifying a structure of interest, and by performing MIP display or the like of the structure of interest by generating a three-dimensional image of the structure of interest from a three-dimensional image including the structure of interest, for example, by using a maximum intensity projection method (MIP method), a minimum intensity projection method (MinIP method), or the like. Alternatively, volume rendering (VR) display of the three-dimensional image is performed, or CPR (Curved Planer Reconstruction) display is performed.
Meanwhile, the aforementioned multi-detector CT can obtain many tomographic images in one operation by plural detectors. Currently, more than 300 slices of tomographic images are obtainable in one rotation. Further, since one rotation of detectors needs about 0.3 second, if images of only a specific organ are obtained, plural three-dimensional images are obtainable in time series with short time intervals. An organ of interest included in the three-dimensional images that have been obtained in time series as described above is displayed in time series. In other words, four-dimensional display, which includes time in addition to the three-dimensional display, is performed. Accordingly, the state of the organ of interest in motion is observable as if a motion image is observed (please refer to Japanese Unexamined Patent Publication No. 2005-322252).
When the three-dimensional images are displayed four-dimensionally, as described above, especially, an analysis of a heart or the like in the field of circulatory organs becomes possible. Further, not only when images of an organ such as a heart or a lung, which has motion, are obtained, but also when three-dimensional images are obtained by using a contrast agent, the flow of the contrast agent is four-dimensionally displayed. Therefore, it is possible to diagnose a specific organ, such as a liver, by the effect of the contrast agent.
When a three-dimensional image is displayed by VR, an organ of interest is extracted. The extracted organ is three-dimensionally displayed by setting a color (R, G, B) and an opacity level (opacity) for the signal value of each voxel based on the signal value (a CT value if the image is a CT image) at each voxel position in the three-dimensional image of the extracted organ. When VR images are four-dimensionally displayed, the VR images are generated by setting colors and opacities for each of plural three-dimensional images, and the generated VR images are displayed in time series.
Meanwhile, when three-dimensional images are obtained by imaging a specific organ in time series with short time intervals, a signal value at a corresponding voxel position of the organ included in each of the three-dimensional images should be the same. However, in actual cases, a signal value at a corresponding voxel position of the same organ often differs from each other by an influence of noise during imaging, or the like. If a signal value at a corresponding voxel position of the same organ differs from each other, as described above, when VR images are four-dimensionally displayed, a color and an opacity at the same position of the organ fluctuate as the organ moves. If the color and the opacity of the organ fluctuate in such a manner, the three-dimensional motion of the position is falsely perceived, and there is a risk of failing to perform accurate diagnosis.
Further, when a series of three-dimensional images in different phases is obtained by imaging an organ such as a heart and a lung, which has motion, in time series with short time intervals, a signal value at a corresponding voxel position of the organ included in each of the three-dimensional images is the same value in many cases. However, when three-dimensional images are obtained in time series with short time intervals by using a contrast agent, the images are obtained to diagnose a temporal change in signal values. Therefore, a signal value at a voxel position of the same tissue included in each of the three-dimensional images differs from each other in many cases. Further, signal values of some tissue are the same in three-dimensional medical images of a certain phase, but different in three-dimensional medical images of another phase. As described above, if the signal value of the same tissue is different from each other in the three-dimensional medical images, depending on the phase, or if the signal values of different tissues are the same, it is impossible to distinguishably display different tissues when VR images are four-dimensionally displayed. If different tissues are not distinguishably displayed, as described above, there is a risk of failing to perform accurate diagnosis.